Conventional piston dampers also include magnetorheological (MR) dampers. MR dampers include a cylinder that contains an MR fluid. An MR piston slideably engages the interior of the cylinder. A first end of a rod is attached to the MR piston and a second end of the rod extends outside the cylinder. The rod and the cylinder are attached to two separate structures to restrain, or dampen, relative motion of the two structures along a direction of travel of the MR piston. Dampening forces are generated within the cylinder of the MR damper that oppose the movement of the rod and the MR piston along the axis of the MR damper.
MR fluids are made up of microscopic metallic particles, a carrier fluid, and stabilizers. The microscopic metallic particles, such as iron, are suspended in the carrier fluid. The microscopic metallic particles in the MR fluid may have a range in size from 0.1 to 10 micrometer and have a shape that appears as ellipsoids or scale spheres when viewed by the human eye upon magnification. If the metallic particles are too large, the metallic particles may settle out from being suspended in the MR carrier fluid. If the metallic particles are too small, the desired magnetic effect on the metallic particles begins to diminish. The typical MR fluid consists of 20-40% microscopic metallic particles by volume of MR carrier fluid. The carrier fluid in the MR fluid may be a type of oil such as hydrocarbon oil, silicone oil, and ester oil. Preferably, hydrocarbon oils are used in MR fluids. Hydrocarbon oils have a desired high lubricity and are available in a wide range of viscosities. Stabilizing agents are additives in the carrier fluid that inhibit the gradual gravitational settling of the metallic particles. If the metallic particles settle in the MR fluid, the stabilizing agent aids to keep the metallic sediment from becoming compacted. If compaction of the metallic sediment occurs, remixing the metallic sediment randomly back within the MR fluid becomes difficult. A low vapor pressure for the MR fluid is also desirable so the MR fluid does not easily evaporate. The MR fluid should be suitable to operate over a wide range of temperatures.
The MR fluid exhibits selectively controlled fluid behavior in the presence of an applied magnetic field. When a magnetic field is not applied to the MR fluid, the microscopic metallic particles are randomly suspended in the MR fluid. In the absence of the magnetic field, the MR fluid has a low viscosity and exhibits liquidity. When a magnetic field is applied to the MR fluid, the microscopic metallic particles form particle chains in the MR fluid within a matter of milliseconds. The particle chains generally align in the direction of magnetic flux of the applied electric field. The formation of the particle chains alters the MR fluid characteristics from a liquid state to a quasi-solid state having increased viscosity and exhibiting behavior similar to a viscoelastic material. A viscoelastic material exhibits both viscous and elastic characteristics when undergoing deformation. Viscous materials resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched, but quickly return to their original state once the stress is removed. The MR fluid, acting as a viscoelastic material in the presence of a magnetic field, exhibits both of these properties. The MR fluid also exhibits time dependent strain characteristics.
In MR damper operation, the MR fluid passes through an opening in the MR piston while the piston travels with reciprocal velocity movement in the cylinder. The MR fluid in the opening is exposed to a varying magnetic field generated by providing a varying electric current to an electrical coil disposed in the MR piston. The piston, formed of a metallic soft magnetic material in conjunction with the electrical coil, forms an electromagnet. The electromagnet applies the magnetic field to the MR fluid in the cylinder. Application of the magnetic field causes the microscopic metallic particles in the MR fluid to form chains in the opening that generally align in the direction of magnetic flux and perpendicular to the flow of MR fluid flowing through the piston. The chains of microscopic metallic particles restrict the movement of MR fluid through the opening in the piston and result in an increase in yield stress of the MR fluid. An increase in yield stress of the MR fluid results in an increase in force in the MR damper. As understood by one skilled in the art, the electric current applied to the electromagnet may be varied to produce varying force operating conditions of the MR fluid at varying piston velocities of the MR damper. Thus, a varied damping effect of the MR piston may be produced. A varied damping effect allows for variably-controlled damping of relative motion between the MR piston and the cylinder to be realized. The damping performance of a MR damper in a suspension system is largely dependent on the force-versus-piston assembly velocity characteristics of the MR damper. The variably-controlled damping performance of the MR damper may be measured and evaluated using a damper force-versus-piston assembly velocity curve.
It is known, according to prior art U.S. Pat. No. 6,612,409 issued on Sep. 2, 2003, and referring to FIGS. 1-2, that a MR damper (10) includes a MR piston assembly with a primary channel used for wire routing (12) disposed in the cylinder (14) about a longitudinal axis A. Hereafter, the MR piston assembly with a primary channel for wire routing will be referred to as a “primary channel MR piston assembly.” A MR fluid (16) is also disposed in the cylinder (14). The primary channel MR piston assembly (12) includes a coaxial piston body (18). A coaxial annular structure (20) about the piston body (18) defines an annular substantially magnetically energizable MR-fluid passageway (22). The piston body (18) further includes a first end (24) and a second end (26) axially spaced from the first end (24). A first end plate (28) is attached to the first end (24) and a second end plate (30) is attached to the second end (26). The piston body (18) defines a coaxial hole (32) therethrough. The piston body (18) includes an outer body surface (34). The piston body (18) defines a circumferential coil groove (36) about the axis A in the outer body surface (34). A primary channel (38) is also defined in the outer body surface (34) from the hole (32) at the second end (26) to the coil groove (36). The primary channel (38) includes a first primary channel portion (39) and a second primary channel portion (39A). The first primary channel portion extends from the second end (26) to the coil groove (36). The second primary channel portion (39A) is disposed in the second end (26) extending from the hole (32) in the second end (26) to the first primary channel portion (39). An electrical conductor (40) is disposed in the hole (32) at the second end (26) and routed through the primary channel (38) and into the coil groove (36). The electrical conductor (40) is further configured to form an electrical coil (42) in the coil groove (36). The hole (32) at the second end (26), the primary channel (38), and the coil groove (36) are filled with a nonmagnetic material (44) to effectively seal the electrical conductor (40) and the electrical coil (42) therein. A coaxial rod (46) is connected at the first end (24) and includes a coaxial opening (48) therethrough. Rod (46) is connected to piston body (18) via retention ring (35) at first end plate (28). The opening (48) includes electrical conductor means for supplying electrical current to the piston body (18) via the electrical conductor (40) and the electrical coil (42). An electrical connection means (50) in electrical connection to a power source external to the MR damper supplies power to the piston body (18) via the electrical conductor (40) and the electrical coil (42). The piston body (18) may further include one or more substantially magnetically non-energizable MR-fluid passageways (52), or by-pass holes, spaced apart from the axis A and outbound from the hole (32). The MR fluid (16), flowing through the cross-sectional area of the magnetically energizable passageway (22) and the magnetically non-energizable passageways (52), is designed so as to meet performance requirements for a specific MR damper application.
The damper force-versus-piston assembly velocity performance curve for the primary channel MR piston assembly in the MR damper depicts MR fluid force data represented along the y-axis and primary channel MR piston assembly velocity data represented along the x-axis. A damper force-versus-primary channel MR piston assembly velocity curve intersects the force axis, or the y-axis, at a y-intercept value above zero when approached from the positive velocity side, and a y-intercept value below zero when approached from the negative velocity side. The wider the difference between the y-intercept value above zero and the y-intercept value below zero results in an increased undesired jump in force between the finite positive and finite negative values. For example, in a vehicle system where the MR damper is employed, the higher the undesired jump in force with each change of movement direction of the primary channel MR piston assembly, a higher value of an undesirable harshness to the vehicle ride may result. The undesired harshness in the ride of the vehicle may be negatively felt by an occupant of the vehicle. Ideally, a damper force-versus-primary channel MR piston assembly velocity performance curve established through the origin (x=0, y=0) is desired to minimize the undesired harshness component from disturbing the smooth ride of the vehicle.
As shown in the prior art in FIG. 1, a single bypass hole (52) defined in the primary channel MR piston assembly assists to lower the y-intercept point closer to the origin and may result in a smoother ride of the vehicle at low primary channel MR piston assembly velocities. However, an inclined ramp portion of the damper force-versus-primary channel MR piston assembly velocity curve subsequent to the y-axis intercept point generally includes at least two distinct inclined ramps segments, or ramp steps. Each distinct ramp step has a unique positive and decreasing slope value. The second step results in lower force capability between the step transition and kneepoint. The two-step incline ramp yields into a knee-point transitioning into a desired, performance curve having a relatively low positive slope at increased force values beyond the knee-point and across the mid-to-high primary channel MR piston assembly velocities.
What is needed is an improved MR damper having a primary channel MR piston assembly that minimizes the two-step inclined force ramp at low MR piston velocities and provides increased force capability.